The present invention concerns an electronically servo-assisted bicycle gearshift and a method for servo-assisting a bicycle gearshift, as well as a program and an electronic circuit having means for carrying out the method.
An electronically servo-assisted bicycle gearshift generally comprises a rear actuator, a front actuator, a means for generating a signal, an electronic control unit, a rear transducer and a front transducer. The rear actuator and front actuator each have a respective motor for displacing a chain through a guide element in an axial direction with respect to a respective gearshift group. Each gearshift group comprises at least two sprockets or toothed wheels associated with the hub of the rear wheel (the sprockets also being called pinions and the chain guide element also being called rear derailleur or simply gearshift) and, respectively, with the axis of the pedal cranks (the sprockets or toothed wheels also being called crowns or gears and the guide element also being called front derailleur or simply derailleur). The chain displacement between sprockets takes place in a first direction (for example from a sprocket with a smaller diameter to a sprocket with a larger diameter, or “upwards gear-shifting”) or in a second direction opposite to the first direction (for example, from a sprocket with a larger diameter to a sprocket with a smaller diameter or “downwards gear-shifting”).
The means for generating a signal requests displacement of the chain from a first sprocket to a second adjacent sprocket of the respective gearshift group, such as levers associated with the two handlebar grips of the bicycle. The electronic control unit is connected to the rear actuator and to the front actuator, and operates, in a normal ride operating mode (i.e. wherein the gearshift is controlled manually by the rider or semi-automatically or automatically by the electronic control unit), to receive the displacement request signal and drive the rear or front actuator, respectively, based upon the displacement request signal to displace the chain from a first sprocket to a second adjacent sprocket of the respective gearshift group, based upon logic positions (“logic values”) representing the physical positions of the various sprockets.
The rear transducer and the front transducer detects the position of the actuators (and therefore of the chain guide elements) and indicates the position to the electronic control unit so that the actuators are stopped when the desired position has been reached.
Electronically servo-assisted bicycle gearshifts are described in U.S. Pat. Nos. 5,480,356; 5,470,277; 5,865,454; and EP 1 103 456, all of which are assigned to Campagnolo S. r. l. and U.S. Pat. No. 6,047,230 and German patent application DE 39 38 454 A1.
In particular, EP 1 103 456 describes a gearshift comprising position transducers of the absolute type, capable of providing an electrical signal indicating the absolute position of the derailleurs. When switched on, such transducers take into account the actual position of the derailleurs, which could be slightly displaced due, for example, to vibrations caused by the travel of the bicycle.
In normal operation, in order to assist a gear-shifting from a first sprocket to a second adjacent sprocket, sometimes it is not sufficient to displace the chain guide element (gearshift or derailleur) up to the second sprocket. In fact, due to the existing distance between the guide element and the second sprocket that the chain must engage, and due to the fact that the chain is at an angle during the gear-shifting, such a movement may interfere with the engagement of the chain on the second sprocket. This is a serious problem when shifting gears.
The problem is particularly serious in the case of the front gearshift, where the chain is taut. To shift gears, in particular during an upwards gear-shifting, the rear actuator or front actuator, respectively, must be displaced to a position typically beyond the position corresponding to the second sprocket. Such a displacement in advanced position with respect to the second sprocket promotes the release of the chain from engagement with the first sprocket and the engagement of the chain on the second sprocket.
In mechanical control gearshifts, a control mechanism acts as an actuator to displace the chain guide element. The control mechanism comprises a steel cable slidably contained in a sheath (“Bowden cable”) between a manual actuation lever and the chain guide element. The actuation of the lever in a first direction applies a traction on the chain guide element through the steel cable, whereas the actuation of the lever in a second opposite direction applies a thrust on the chain guide element through the steel cable, or lets the cable and the chain guide element free to be returned by a return spring.
To make gear-shifting easier, some mechanical control gearshifts use a control system in which the actuation of the control lever causes the steel cable to move by such a length that the chain guide element moves further than necessary to reach the position of the adjacent sprocket. When the control lever is released, the return spring acts to take the steel cable—and thus the chain guide element—back to the position corresponding to the second sprocket. In other words, the actuation of the control lever causes a temporary displacement of the chain guide element greater than the pitch between two adjacent sprockets of the gearshift group by a certain amount indicated hereafter as “overstroke.”
Such a mechanical control gearshift unfortunately has some drawbacks. First, it requires periodic and precise mechanical adjustment of both the steel cable and the return spring tension. Second, it is possible to adjust the amount of the overstroke to only a single value, which impacts as much in all of the upwards gear-shiftings as in all of the downwards gear-shiftings. Consequently, when the amount of the overstroke in a gearshift group is adjusted for optimal gear-shifting in one direction (for example for upwards gear-shifting), a gear-shifting in the opposite direction (in the example, downwards gear-shifting) is unsatisfactory. Alternatively it is necessary to adjust the amount of the overstroke to an intermediate compromise value, obtaining sufficient but not optimal results in gear-shiftings in both directions.
In the case of the front gearshift group, upwards gear-shifting to the outermost sprocket (the sprocket with the largest diameter) is difficult. In this case even the provision of the overstroke may not be sufficient to ensure that the chain engages correctly. Particularly experienced riders could avoid this by keeping the front derailleur at the overstroke position for a certain amount of time (by holding the control lever pressed). The time spent in overstroke position could however be only determined “by ear” and/or “by sight” by the rider, with the result that it could be too brief to give the desired result or so long as to cause harmful stresses to the mechanics of the gearshift or even the arrangement of the chain in positions such as to cause dangerous falls.